The coal seams in the Highveld Coalfield are generally described as flat lying to gently undulating, with a regional dip to the south. The depth of the coal seams increases in a southerly direction. The Highveld coalfield has been affected by numerous dolerite dykes and sills related to the Drakensberg Formation flood basalts. Within the vicinity of the dolerite, the usual structural complications and devolatilization of the coal occurs.

At surface, the soils are described as sandy loams to sandy clay loams. Towards the south, the soil type becomes a dark grey sandy clay, due to the weathering of the underlying dolerite sill. Any outcrops or sub-crops are dominated by sedimentary rocks of the Ecca Group (Vryheid Formation).

The depth of weathering ranges from less than 1.0 m to almost 30 m in a few isolated holes located within the low-lying areas. The weathered zone predominantly comprises a grey-brown sandstone, with minor clay rich intercalations. The impact that the deeply weathered areas may have on the stability of the highwall within the current opencast areas has been reviewed. The resistivity data derived from the Dighem survey was compared to the point data from the drill holes in order to produce a continuous, calibrated surface representing the interface of soft and hard interburden. This surface was included in the geological model to allow a more precise estimation of volumes of the various materials.

In order to model the weathering surfaces more accurately, Isibonelo has created a short term geological model which is updated at site on a regular basis. The model is updated with information obtained from face mapping, geophysical logs, and drill holes.

Pre-Karoo Basement
The glacially sculpted pre-Karoo basement comprises metasedimentary and metavolcanic rocks of the Paleoproterozoic Waterberg Group and post-Waterberg intrusives (felsites and granites). Due to the majority of the drill holes being drilled to just below the No 2 Seam, very few holes (2.5%) intersected basement at Isibonelo. Where the basement was intersected, it has been described as a light red, fine-grained granite.

Ecca Group
Within the Ecca Group, the Vryheid formation comprises zones of alternating conglomerates, sandstone and mudstones within which the coal seams are located. The stratigraphy of the Vryheid Formation may be divided into three main sequences at Isibonelo; a basal No 2 Seam Sequence, overlain by the No 4 Seam Sequence, which in turn is overlain by the No 5 Seam Sequence.

No 1 Seam Sequence
The Number 1 Seam (No 1 Seam) is only sporadically developed at Isibonelo. It has not been included in the geological model and does not form part of the resource base.

No 2 Seam Sequence
The No 2 Seam sequence includes the succession from the top of basement to the top of the No 2 Seam and incorporates the rocks of the Dwyka Group, as well as the overlying No 1 and No 2 coal seams.

At Isibonelo, the rocks of the Vryheid formation lie directly on top of the Dwyka Group, comprising both diamictites and varved shales. The clastic interval below the No 2 Seam comprises a fine to course-grained sandstone, with the occasional granulestone/gritstone at the base of the succession.
This upward fining sequence from the top of the Dwyka to the base of the No 2 Seam, where intersected, is in the region of 25 m thick, increasing to over 40 m towards the east (South Pit).

The No 2 Seam, where developed, is on average 1.25 m thick, and comprises a coal and carbonaceous mudstone rich interval. As such the No 2 Seam is not considered economic and does not form part of the resource base.

No 4 Seam Sequence – Target Seam
The No 4 Seam Sequence comprises the succession from the immediate roof of the No 2 Seam to the top of the No 4 Seam. The immediate floor of the No 4 Seam predominantly comprises a fine to medium grained sandstone that is sometimes bioturbated and often contains siltstone laminations.

The No 4 Seam is well developed across the entire Isibonelo licence area and is the sole economic target. The seam is on average 5.6 m thick, ranging from 4.2 m to almost 7 m in places. The seam is divided into three zones, based on the lithological variation of the seam. The zones are named from the base up as the Number 4 Seam Lower Zone (S4Z1), Number 4 Seam the Middle Zone (S4Z2) and the Number 4 Seam Upper Zone (S4Z3). The Lower Zone is the thickest and best quality of the three zones (21.7% average ash content); the Middle Zone is generally a poor quality coaly shale with multiple mudstone partings (45% average ash content), and the Upper Zone is a dull-lustrous coal with shaley lenses and occasional bright bands (31% average ash content). Currently the full No 4 Seam (S4Z1+S4Z2+S4Z3) is mined.

No 5 Seam Sequence
The No 5 Seam Sequence comprises the succession from the immediate roof of the No 4 Seam to the top of the No 5 Seam.

The No 5 Seam’s extent is limited by the depth of weathering, as the seam ranges from 15 to 23 m below surface. The seam is on average 1.8 m thick, and as per the No 4 Seam, is divided into three distinct lithological zones, namely the Number 5 Seam Base (S5B), Number 5 Seam Parting (S5P) and Number 5 Seam Top (S5T) from the base up. The S5B is the lower most zone; it averages 0.78 m in thickness and comprises a dull-lustrous coal, with bright bands, and an occasional in-seam carbonaceous shale parting. The S5P is a finely laminated carbonaceous shale, and has an average thickness of 0.64 m. The S5T is on average 0.36 m thick and comprises a relatively bright coal with an abundance of calcite on the cleats of the coal. The full No 5 Seam has an average ash content of 42%.

Isibonelo is an opencast colliery using draglines and truck and shovel methods to exploit the coal within the Mining Right; the coal is solely dedicated to the SSF facility. The southern portion of North Pit is too deep for surface extraction and these potential underground resources are being ceded to Sasol for exploitation from the Sasol Syferfontein Shaft. The terms of the supply agreement with Sasol dictate the LoM plan. Isibonelo commenced operations in 2003 where the existing opencast equipment and some portions of the Kriel South resource (owned by the Company at that time) were swapped by the Company in return for an ongoing dedicated supply contract.

This was intended to supply Sasol with 5 Mtpa for at least 20 years (i.e. 100 Mt). This deal was rationalised in 2019 to account for unplanned cost and resource constraints.

The main target coal seam is the No 4 Seam, although the No 5 Seam.

The open pit is designed for dragline operations and in the case of the North Pit the dragline is assisted by a prestrip truck shovel operation. The blocks are designed at 60 m wide by 150 m long.

The mining method in the North Pit is designed to mine a combined total thickness of approximately 65 m which is split into approximately six metres of soft overburden material mined with a hydraulic excavator along with 16 m of hard pre-strip material that is mined with a rope shovel after drilling and blasting. The remainder is mined by the dragline in a side-casting method.

The South Pit layout is fully utilized, but the North Pit is limited to the length of the initial strip, which is extended by approximately 20 m in each subsequent strip to lengthen the strip without creating a boxcut to recover coal in the Zimele area. The main motivation for this is that the cost of digging a boxcut cannot be supported by the mine economics.

Isibonelo Colliery does not have a coal dense medium washing plant, only a crush-and-screen facility. The plant receives coal from the mine at the RoM tip, where the coal is screened and crushed to ensure that the sizing specifications are met.

The coal is hauled from the mine pit and back-tipped into a 450 t bin at the Rom tip, above which is a static grizzly with 1 m apertures. The coal passes onto an apron feeder which discharges at a rate of up to 1 600 tph. From the apron feeder the coal is fed onto a scalping screen with 100 mm apertures. The plus 100 mm coal is crushed in a mineral sizer, crushing to a nominal 300 mm, while the minus 100 mm coal passes directly to the tip clearance conveyor.

The coal discharged from the mineral sizer is fed to a secondary mineral sizer for crushing to a nominal 100 mm. When the mine began this was the complete RoM circuit, but it was discovered that there was too much plus 100 mm coal in the product. A further stage was ad ........